Use of molecular resonance in a gas



June 1955 w. D. HERSHBERGER USE OF MOLECULAR RESONANCE IN A GAS 2 Sheets-Sheet l INVENTOR ATTORNEY June 28, 1955 Filed Jan. 8, 1948 W. D. HERSHBERGER- USE OF MOLECULAR RESONANCE IN A GAS 2 Sheets-Sheet 2 v ei n g g N \Q- g- -I\ d L3 4 '3 3 N E w R Q j 1: l v -gg &

5 I5 55; a? 3* $1 8 3 z INVENTOR Wzlnazflf/srshbtg'er ATTORNEY USE OF MOLECULAR RESONANCE IN A GAS William D. Hershberger, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 8, 1948, Serial No. 1,240

Claims. (Cl. 250-36) This invention relates to the utilization of the molecular resonance of certain gases to obtain frequency-determining elements having high selectivity or high Q at discrete ultra-high frequencies and particularly relates to the stabilization of the frequency of microwave oscillators.

Heretofore many methods and systems have been proposed for stabilizing the frequency of microwave oscillators but in general they have not afforded the accuracy of frequency or the stiffness of frequency control obtainable at low and medium radio-frequencies with piezo-electric crystals.

The microwave absorption spectra of certain gases, in-

cluding ammonia, carbonyl sulphide and methyl halides,

comprises lines of distinctive and different frequency distribution for the different gases. At very low pressures, in the case of ammonia, each of these lines breaks up into a plurality of still more sharply defined lines, each corresponding with a definite frequency which is independent of such ambient conditions or factors as temperature and pressure and which, so far as has been determined, can be varied only by subjecting the gas to a relatively strong magnetic or electric field. Methods of and systems for frequency-stabilizations which involve displacement of the absorption lines by strong fields are not herein claimed, but are claimed in my copending application Serial Number 5,563, filed January 31, 1948.

In accordance with one aspect of the present invention, the molecular resonance exhibited by such gas is utilized to exert a pulling effect upon the frequency of oscillations generated by a magnetron, klystron or other microwave generator incorporating or used with a resonant cavity or equivalent. As the frequency of the generated oscillations tends to rise above or drop below the desired frequency, the reactance of a body of the gas as reflected into the tube cavity rapidly changes and in sense to restore the frequency to the desired magnitude. Thus, the body of gas, in rough analogy to a piezo crystal, is the electrical equivalent of a high-Q, two-terminal network forming the frequency-determining element of the oscillatory system. In this respect, the present application is in part a continuation of my copending application Serial No. 596,242, filed May 28, 1945 (Fig. 6) in which frequency-control is ancillary to gas analysis.

In some forms of the invention, the container for the microwave absorptive gas is itself relatively broadly resonant at the desired operating frequency of the oscillator so that, in combination with the absorptive gas, it provides a circuit element whose Qis extremely high at the resonant frequency and which possesses substantial reactance throughout an appreciable range above and below the resonant frequency.

In another form of the invention, the container for the microwave absorptive gas is itself resonant but its reactive effects upon the oscillator are cancelled by the reactance of another cavity, or equivalent, free of gas and also coupled to the oscillator. The resonant container of chamber for the gas, so far as the oscillator system is concerned, is aperiodic or non-resonant so that the gas alone serves as a sharply resonant element for maintaining constant the frequency of the generated oscillations.

2,712,068 Patented June 28, 1955 In any of the foregoing forms, the stabilized oscillator may be used for delivery of power to a load or it may be used as a signal generator suited for use as a highly precise frequency standard.

Further in accordance with the invention, the high-Q circuit elements above described may themselves be used as wavemeters of high precision suited to check whether the frequency of generated oscillations corresponds with any of the discrete frequencies corresponding with absorption lines of one or more gases exhibiting the characteristic of molecular resonance.

The invention further resides in the methods and in the.

' systems hereinafter described and claimed.

For a more detailed understanding of the invention and for illustration of systems embodying it, reference is made to the accompanying drawings in which:

Figures 1 to 4A inclusive are curves referred to in explanation of principles involved in the invention;

Figure 5 diagrammatically illustrates a system for stabilization of a magnetron in which the stabilizing element. is a resonant cavity containing gas exhibiting molecular resonance;

Figure 6 diagrammatically illustrates a system similar to. that of Figure 5 with provision for compensation of the reactance of the gas-containing cavity;

Figure 7 is a chart indicating certain microwave frequencies for which specified gases exhibit molecular resonances;

Figure 8 diagrammatically illustrates another form of oscillator system suitable for lower frequencies and employing molecular resonance for frequency stabilization; and

Figures 9 and 10 illustrate two forms of wavemeter utilizing a molecular-resonant gas as a frequency standard.

In explanation of principles underlying the invention,

it is known that there are a number of gases including,

\ ammonia (NHs), COS, CHaOH, CI-IsNI-Iz, NHzD, NHD2 and ND3, which exhibit selective absorption in theultrahigh frequency portion of the frequency spectrum. From measurements of the resonant frequency of such a gas, it is known the magnitude of the absorption co-efficient may be quite independent of the gas pressure but that the apparent width of the absorption line decreases, throughout a substantial, range of pressure, with reduction: of pressure. Specifically, at wavelength of 1.25 centimeters (24 kilomegacycles), the Q of the ammonia line isapproximately 10 when the gas pressureis of an atmosphere and is at A of an atmosphere. However, as the pressure. is further and further reduced to, for example, the order of millimeters of mercury, the absorption region breaks up into a plurality of sharply defined component lines comprising the rotational fine structure of the original absorption, each precisely corresponding with a particular frequency and unaffected by any known factor except a strong electric or magnetic field.

This sharply resonant effect of the gas may be utilized in a microwave circuit for stabilization or precise measurement of a discrete frequency corresponding with a selected one of these sharp lines by employing a Waveguide or a resonant cavity as a chamber for the gas.

For the moment it is assumed that such cavity, or equivalent, has no losses at all, that is, has an infinitely large Q and is filled with ammonia gas at atmospheric pressure: under this circumstance, the gas is non-resonant and absorbs microwave energy over a relatively where A is equal to 1.25 centimeters.

, Such a gas, placed in a cavity with an initial Q of 5,000, gives a net Q of 2,500 because 1 1 l TQI QQ where Q1 equals the Q of the gas and Q2 equals the Q of the cavity.

When, however, the gas absorbs microwaves not over a wide range of frequencies but over a narrow range which can be controlled, as above stated, by varying the gas pressure so that the absorption line breaks down into a plurality of sharply-defined discrete lines, the situation is entirely different. For example, at a pressure of 0.02 millimeter of mercury, the half-width points of the resonant curve of the cavity-gas combination, corresponds was a Q of 40.000, and at still lower pressures, Qs as high as 100,000 may readily be obtained.

The resonance curve of a circuit element comprising a highly-resonant gas inside of a cavity resonator is of the character shown in Figure 1 having a pronounced dip at the resonant frequency F of the gas and the cavity. The reactance of the cavity alone, without the gas. is exemplified by curve X, Figure 2. As there shown, as the frequency is increased, the reactance of the cavity increases rather slowly to a maximum value at a frequency F1 appreciably lower than the resonant frequency F, and then decreases somewhat more rapidly passing through zero value at frequency F: as the frequency is still further raised, the reactance of the cavity increases, in reverse sense, to a maximum at a frequency F2, somewhat higher than the resonant frequency F, and thereafter more slowly decreases. The cavity exhibits maximum inductive reactance at frequency F1 and maximum capacitive reactance at frequency F2 both well displaced from the resonant frequency F. p

In contrast with the relatively slow change of reactance of the cavity between the frequencies F1 and F2, reference is made to Figures 3 and 3A which show that the reactance X1 or X2 of the gas itself attains maximum values at frequencies F3 and F4, both extremely close to the resonant frequency F and rapidly changes in magnitude between those frequencies. The reactance curve X2 of Figure 3A is the inversion of Figure 3 as effected for a transformer, for example.

The composite reactance of the gas-filled cavity is exemplified by curve (X, X1) of Figure 4 or curve (X, X2) of Figure 4A, and, as there shown, the reactance increases very rapidly in amplitude for minute deviation of the frequency in either sense from the resonant frequency F, the extreme steepness of the curve in the vicinity of resonance being due to the high Q of the gas. In either case, the reactance/frequency characteristic of the gas-filled cavity ideally suits it as a stable high-Q circuit element.

In the microwave generator system shown in Figure 5. the sharply resonant characteristic of a resonant cavity tuned to a desired operating frequency and containing a gas exhibiting molecular resonance at that frequency is utilized to stabilize the frequency of a multi-cavity magnetron 10. The cavity 11 which contains the body of gas 12 is suitably coupled to the magnetron to appear thereto as a highly reactive load capable of pulling the frequency of oscillations generated by the magnetron to the desired operating frequency. Such pulling effect provides the frequency-control obtained in the gas-analysis system of Fig. 6 of my aforesaid copending application Serial No. 596,242, The pulling figure of merit of the tube can be determined by known techniques. Pulling" has been arbitrarily defined as the maximum frequency change induced in a microwave generator by use of a reactive load which sets up a 1.5 to 1 voltage standing ratio; the phase of the reflection co-efiicient is varied 180 and the frequency noted, while the standing wave ratio is held constant. With this information obtained, the distances a and b, Figure 5, may be chosen for maximum utilization of the pulling characteristic of the magnetron. for

iii.

frequency stabilization by the sharp resonance characteristic of the gas-filled cavity 12. The distance b is measured from the junction of the magnetron and the waveguide 9 to the junction of the cavity 11 and the waveguide 9; the distance a is measured from the latter junction to the window 17. Adjustment of lengths a and b may de sirably be made while monitoring the output frequency of the tube with a swept monitor or spectrum analyzer during application of a modulating voltage to oscillator tube 10. Optimum adjustment is that which results in minimum shift of the carrier frequency.

When a resonant cavity, or equivalent, is used to permit utilization of molecular resonance of a gas in stabilization or determination of the frequency of an oscillator, saturation effects are to be avoided. Such effects may occur when the intensity of the radio-frequency fields in the cavity is excessively high and make themselves evident by apparent reduction in absorption as the field intensity is increased. When difficulty with such effects is experianced, recourse may be had to high-mode cavities using a correspondingly larger sample of the gas and in which there is greater distribution of the radio-frequency energy so that its field intensity is nowhere excessively intense.

Resonant cavities filled at low pressure with a microwave absorbing gas may be used in a waveguide system either in series" or shunt connection as those terms arc understood in microwave techniques.

When, as in Figure 5 the stabilizing element is a resonant cavity and a molecular resonant gas, the resulting impedance (or admittance) includes, as components, the resistances (or conductances) of the gas and of the resonator and the reactances (or susceptances) of the cavity and the gas. The reactive (or susceptance) effect of the cavity may be eliminated, leaving only the resonant effects of the gas by use, Figure 6, of a second resonant cavity 14 identical with cavity 13 but which is not filled with the microwave absorptive gas and is spaced from cavity 13 by a distance 0 such that the reflection at the junction of cavity 14 with the waveguide 9 cancels out the reflected wave at the junction of cavity 13 and the waveguide 9. In short, distance 0 is so chosen or adjusted, as by telescoping sections. that in the absence of the absorptive gas in chamber 13, the guide is matched at the operating fre-. quency for maximum power transfer from magnetron 10 to a matched load 16. The tuning screw 15, or equivalent, may be utilized for fine adjustment of electrical length c. When a microwave absorptive gas is subsequently disposed in the cavity 13, there effectively remains only the impedance or admittance component due to gas resonance, and hence the stabilizing effectiveness of the twochamber arrangement of Figure 6 is that of a circuit element having a high-Q which may be of the order of l00,000 or more.

In each of the foregoing arrangements, it shall. of course, be understood that the chamber for containing the microwave absorptive gas is provided with a closure, such as window 17, which confines the gas but is substantially transparent to the microwave energy from the generator. Windows 17 may, for example, be of thin mica.

in general, the gases suitable for use as precise frequency standards or for stabilization of microwave generators are those having a dipole moment including, for example, the gases above specifically mentioned. This class of gases, at suitably low pressures of the order of l0- millimeters of mercury, afford a substantialnumber of discrete frequencies at which. by recourse to the present invention, an oscillator may be stabilized. At suitably low pressures, ammonia gas alone, Figure 7, has a substantial number of sharp'absorption lines in the range of from 19.5 to 25.5 kmc., any one of which may be used as above described as a frequency-standard for oscillator stabilization. Gases other than ammonia which exhibit molecular resonance may, of course, be used; for example, there,ar e pure rotational transitions for COS at. 2.5. and 1,25 centimeters, for CH3 at 6 .milimeters for CHaCl at 1.06 and 0.53 centimeters, and for CI-IsBr at 1.53 and 0.765 centimeters. From the chart shown in Figure 7, it is apparent that even a few gases afford a large number of standard frequency lines scattered over a substantial range of microwave frequencies.

Materials such as NHzD, NHDz and ND3 exhibit resonance at wavelengths as long as 1 meter or frequencies as low as 300 megacycles. With the latter named gases exhibiting molecular resonance at the lower microwave frequencies, the frequency-stabilizing element may be, as shown in Figure 8, a suitable length of gas-tight concentric line 13 filled with one of these gases and connected to a conventional triode or other tube 19. In the particular arrangement shown in Figure 8, the oscillator is of the tuned-grid, tuned-plate type and the tuned-grid circuit which serves as the high-Q frequency-determining circuit of the oscillator consists of the concentric line 18 filled with the resonant gas. The remainder of the oscillator circuit is conventional and need not be specifically described. It shall, of course, be understood that other conventional oscillator arrangements using resonant cavities, concentric lines or waveguides which may serve as containers or cells for the molecular resonant gas may be stabilized as to frequency by recourse to the principles above explained and generally in accordance with the specific examples given.

Any of the oscillators so stabilized may be used to supply power to a load, for example, to an antenna, or may be used as highly precise standards of frequency in otherwise conventional methods of frequency measurement. Moreover, a resonant cavity 20, Figure 9, or a length of waveguide 21, Figure 10, filled with a gas exhibiting molecular resonance at the fundamental or higher modes of the cavity or guide may be used as an absorption wavemeter for adjusting an oscillator to operate at a molecular resonance frequency of the gas or to check whether the frequency of generated oscillations corresponds with that of the precision wavemeter. A set of said precision wavemeters or frequency standards for a large number of discrete frequencies may be made by suitable choice of the gas and of the dimensions of the associated cavities. Each wavemeter unit, Figures 9 and 10, may be provided with a crystal rectifier, or equivalent, for connection to a sensitive meter 22, or equivalent device, for indicating or recording the response of the wavemeter to impressed oscillations.

From the general rules explained and the specific examples given, it should be apparent to those skilled in the art how other gases exhibiting molecular resonances may be utilized to determine the frequency of microwave energy.

What is claimed is:

1. A microwave oscillator system for generating oscillations of highly precise frequency comprising a microwave generator including a resonant cavity, a transmission line from said cavity, a pair of resonant cavities connected to said transmission line at such points their reactive effects mutually cancel as reflected into said first-named cavity, and a gas in one only of said pair of resonant cavities and exhibiting sharp molecular resonance at said frequency.

2. A microwave oscillator system for generating oscillations of controlled frequency comprising a microwave generator, a first cavity containing a body of gas exhibiting molecular resonance and broadly resonant over a range including the molecular resonance frequency of the gas, a transmission line coupling said first cavity to said generator for impression of the generated oscillations on said gas and for reflection back to the microwave generator of the reactive effects of the gas as the frequency of the generated oscillations deviates from the molecular resonant frequency of the gas, and a second broadly resonant cavity coupled to said line substantially to neutralize the reactance of said first cavity whereby the gas alone serves as a sharply resonant element for stabilizing the frequency of the generated oscillations.

3. A microwave arrangement comprising a microwave oscillator including a resonant cavity, a chamber confining at low pressure a gas exhibiting molecular resonance at a frequency in the generation range of said oscillator, and a wave transmission path between said oscillator and said chamber for transmitting to the gas microwave energy generated by said oscillator and for re-transmitting over said same path from said gas-chamber to said resonant cavity of the oscillator a control effect for effecting control of the frequency of the generated oscillations by the variations with frequency of the reactance effect of said gas.

4. A microwave arrangement comprising a microwave oscillator including a resonant cavity, a chamber confining at low pressure a gas exhibiting molecular resonance at a frequency in the generation range of said oscillator, said chamber being broadly resonant at said frequency, and a wave transmission path between said oscillator and said chamber for transmitting to the gas microwave energy generated by said oscillator and for re-transmitting over said same path from said gas-chamber to said resonant cavity of the oscillator a control effect for effecting control of the frequency of the generated oscillations by the variations with frequency of the combined reactive effects of said gas and said broadly resonant chamber.

5. A microwave arrangement comprising a microwave oscillator including a resonant cavity, a non-resonant chamber confining at low pressure a body of gas exhibiting molecular resonance at a frequency in the generation range of said oscillator, and a wave transmission path between said oscillator and said chamber for transmitting to the gas microwave energy generated by said oscillator and for re-transmitting over said same path from said chamber to said resonant cavity of the oscillator a control effect for effecting control of the frequency of the generated oscillations by the variations with frequency of the reactance effect of said gas.

References Cited in the file of this patent UNITED STATES PATENTS 1,929,878 Clavier Oct. 10, 1933 2,140,339 Travis Dec. 13, 1938 2,349,440 Lavoie May 23, 1944 2,404,568 Dow July 23, 1946 2,439,388 Landon Apr. 13, 1948 2,444,041 Harrison June 29, 1948 OTHER REFERENCES Ph.D. Thesis entitled Ammonia Absorption Measurements etc. by H. S. Howe, submitted to the University of Michigan, 1940. 

1. A MICROWAVE OSCILLATOR SYSTEM FOR GENERATING OSCIL LATIONS OF HIGHLY PRECISE FREQUENCY COMPRISING A MICROWAVE GENERATOR INCLUDING A RESONANT CAVITY, A TRANSMISSION LINE FROM SAID CAVITY, A PAIR OF RESONANT CAVITIES CONNECTED TO SAID TRANSMISSION LINE AT SUCH POINTS THEIR REACTIVE EFFECTS MUTUALLY CANCEL AS REFLECTED INTO SAID FIRST-NAMED CAVITY AND A GAS IN ONE ONLY OF SAID PAIR OF RESONANT CAVITIES AND EXHIBITING SHARP MOLECULAR RESONANCE AT SAID FREQUENCY. 